Photodetectors convert modulated light into electrical signals. Photodetectors are used in many areas including, but not limited to, antenna remoting, spacecraft RF interconnects, analog and digital links, and other applications using a fiber optic link having a high spur-free dynamic range.
Solid state photodetectors typically operate by incident light or photons being absorbed to create electrons and holes, known as carriers. A device such as a p-i-n (PIN) diode sweeps away the carriers to electrical contacts to produce a photocurrent. A PIN diode is formed when p-type and n-type semiconductor regions are separated by a layer of intrinsic semiconductor material.
An example of a prior art surface coupled PIN photodetector 100 is shown in FIG. 1. The goal in a surface coupled PIN is to absorb as much of the light in as short a distance as possible. The entire absorbing region is made from a narrow-bandgap material, which is covered with highly doped material, forming ohmic junctions that are transparent to both electrons and holes.
Conventional surface-coupled PIN photodetectors have a small surface area and a large bandwidth, but are limited in the amount of optical power they can absorb. Excessive power may damage the photodetector, and produces saturation effects that reduce performance. It is possible, by making the surface area of the device larger, so that the optical power can be spread out over a large area, to reduce the damage and increase the saturation threshold. However, this reduces the RF bandwidth of the device by the corresponding increase in capacitance. A simple PIN photodetector is not capable of unlimited high-power operation and large bandwidth.
Presently, a traveling-wave photodetector 200 having a larger surface area is known in the prior art and is shown in FIG. 2. The photodetector 200 has a thin absorbing layer 202 to achieve low absorption per unit length. The layer 202 is positioned between two non-absorbing layers 204. However, the thin absorbing layer has a significant drawback. The electron-hole pairs that are generated within the layers become trapped and can only leave the layers by way of a thermal process. The thermal process is inherently slow, which means that any photodetector made using thin layers will have an upper frequency response that cannot exceed a few Gigahertz. It has been suggested to use compositional grading. While this process alleviates the trapping problem, it does not eliminate it.